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screening to below the top of the water table), because noil gas will not be properly transported. The soil vapor and monitoring well headspace were collected through a septum with a microsyringe and injected into a Photovac IOSSO gas chromatograph for analynin. The Photovac 1OSSO is a portable, prograrrmble, integrating gas chromatograph with a photoionization detector (PID) • The PID in a nondestructive flow-through detector that uses high energy f ultraviolet radiation as its ionization source. Vapor samples are injected ante- the gas chromatograph, separated on an analytical column, and sensed by the detector. The high energy radiation ionizes compounds, generating an energy increase in the detector which appears ab an electrical signal, measured in Volts: this is integrated across time by the instrument to give a value for the peak in Volt-seconds (V-sec) . The instrument is -a. calibrated with standards consisting of 10 pM benzene and 10 ppm toluene in air. Far the reading of each stendard, the instrument stores the known concentration and the V-sec r:-osponse to it. The ratio for the standard, V-sec:ppm, is then used to quantify the concentrations of identifiable vapors in field souple:s according to their V-sec values. The concentrations of unidentified compounds are calculated in a similar manner. Although petroleum hydrocarbons produce'variable instrumental responses, the assumptipn may be made that all of the hydrocarbon constituents have response-to-concentration ratios approximately equivalent to that of benzene and that all q-aantifications may be based on the ratio :or benzene. In the tablt of results, the column entitled "Peaki prior to Benzene" represents the sum of the responses In v-sec for all peaks eluting prior to benzene:, proportioned to the calibrated V-ssec response for benzenes. Similarly, the column entitled *Total Detected Petroleum Hydrocarbons" or "Total Volatile Hydrocarbons" 4